In the field of sensors designed to produce digital images, the prior art contains two different principles for converting the electromagnetic rays to electrons and, hence, to digital signals. The first of these two principles, called “indirect conversion”, consists in converting the X-rays to visible light photons in a scintillator material, and then in converting the visible light photons to electrons by means of photodiodes prepared on boards of half-conductive material, such as amorphous silicon.
However, in a manner known per se, the sensors operating according to this first principle have limited performance in terms of resolution and sensibility to low exposure dose. In a manner known per se, the resolution is defined as the capacity of a sensor to distinguish two points—objects in the digital image produced. The sensitivity is defined in a manner known per se as the capacity of the sensor to detect a certain quantity of incident photons.
The second conversion principle, developed and implemented more recently, is called “direct conversion”. The sensors operating according to this second principle are required to have superior performance to the previous sensors, in particular a finer resolution and a higher sensitivity to a low dose.
Since this is the fundamental performance of a sensor, any improvement to this performance is likely to concern all fields in which sensors are used to produce digital images, in particular in the fields of medical imaging, scientific and space instruments or nondestructive testing.
This direct conversion principle is described here in relation to FIG. 1, which is a schematic representation of a cross section of a prior art sensor. Direct conversion is achieved regardless of the frequency range of the incident electromagnetic radiation, of the X or γ type.
The direct conversion sensor shown in FIG. 1 is prepared and operates by the technique called “flip chip”. The incident electromagnetic rays are converted directly to electrons in the sensing layer 11, which is generally prepared from materials having a high atomic density, such as CdTe, or lead or mercury halides, such as PbI2 and HgI2, etc. The electrons thus generated by the interactions of the incident photons with the sensing layer 11 form electric currents, whereof the strength is proportional to the energy and to the number of incident photons. These electric currents are collected and flow in conductive pads 15 to travel up to the read circuit in order to be quantified and hence “digitized” therein.
In a manner known per se, each of the conductive pads 15 corresponds to a pixel of the matrix forming the digital image. The conductive pads 15 are consequently distributed discretely and uniformly in the two planar dimensions of the sensor.
Between each of the pads 15 and the sensing layer 11, an electrode 16 is generally placed, made from a conductive material such as gold, suitable for effectively collecting the charge carriers, electrons or holes, issuing from the interactions between photons and matter.
The conductive pads 15 are each mounted on a conductive pad 13, for example aluminum, whose role is to transmit the collected currents to the read substrate 12. The read substrate 12 may comprise a board made from amorphous or polycrystalline silicon, or even an “Application Specific Integrated Circuit” (ASIC), or of the CMOS semiconductor type. The electric currents may then be read and used in a manner known per se by the read circuit to produce a digital image representative of the scene observed.
Furthermore, the conductive pads 15 are made from an adhesive material having an isotropic conductivity. Such an adhesive generally consists of a polymer matrix including uniformly distributed metallic charges, whose role is to provide the electric conduction.
The adhesive that forms the pad 15 also has the function of providing the mechanical cohesion between the pads 13 of the read circuit and the sensing layer 11, generally provided with electrodes 16. Thus, the sensor shown in FIG. 1 is prepared by the mutual mating and bonding of two planar components by means of a multitude of adhesive points distributed in a regular grid corresponding to the arrangement of the pixels of the matrix of the sensor.
During the fabrication of the sensor, the conductive pads 15 are disposed at each of the pads 13 of the read circuit, and the sensing layer 11 is then added on to the conductive pads, the overall assembly then undergoing a treatment for curing of the adhesive pads 15.
The structure of the sensor shown in FIG. 1 therefore serves, in addition to the mechanical mating discussed above, for the integral transmission of the signals or electric currents from the sensing layer 11 to the read circuit 12-13. In the context of the present invention, “integral” means a transmission that takes place without inter-pixel leakage current, that is, without leakage current between neighboring conductive pads 15.
In fact, in a continuous planar structure like the one of the sensing layer 11, charge carriers exist, directed toward the neighboring pixels of the interaction site instead of traveling toward the nearest pixel or conductive pad 15. In a manner known per se, these inter-pixel leakage currents or leakage currents within the sensing layer 11 cause a degradation of the image quality, that is, a lowering of the ratio of the coherent signal to the undesirable noise. This degradation must, whenever possible, be offset by a complex and therefore costly image processing.
The structure described with regard to FIG. 1 therefore has the advantage of isolating the conductive pads 15 from one another, hence are preventing the leakage currents at their level. Thus, the electrons which reach the electrode 16 are “captured” by the adjacent conductive pad 15 thereby tending to maximize the signal-to-noise ratio of the digital image.
However, the step of fabrication of the sensor consisting in discretely positioning each of the pads of conductive adhesive 15 requires especially high accuracy as the desired resolution is finer.
Moreover, it is important to deposit an identical quantity of adhesive material for each of the pads 15 in order to guarantee an effective electrical connection between the parts to be mated. The batching of this quantity of material must therefore be very precise.
In fact, the precision required for the batching and positioning of the pads 15 often proves to be difficult to obtain and always sharply increases the production cost of the sensor, regardless of the technique employed (screen-printing, dispense, etc.).
Furthermore, depending on the adhesive material selected to prepare the pads 15, the speed of deposition of all the pads 15 may exert some influence on the quality of the bonding, and hence on the electrical conduction. This speed represents an even more critical parameter as the dimensions of the sensor are larger.
Moreover, this type of connection does not allow for sufficient overall mechanical strength.
In the context of the electrical interconnection of two components, document U.S. Pat. No. 6,396,712 describes the use of a conductive layer having isotropic resistivity simultaneously performing the functions of interconnection and mating of said components. The possibility of repair is mentioned, using as an electrically conducting layer, a thermoplastic material having a well known low adhesive capacity.
The application of the teachings of this document is therefore confronted with major difficulty for the full control of the uniformity of the conduction and mating properties.
Document JP5-167057 describes an electromagnetic wave detection device comprising a sensor made from CdTe prepared on a substrate transparent to the radiation to be detected, and provided with continuous electrodes on each of its faces, respectively made from CdS and from a-Si:H, that is, of amorphous silicon, and respectively playing the role of an electron stopping layer and a charge accumulation layer. The latter is electrically conductive in order to permit the transfer of the charges to the read circuit via the electrodes and the discretely distributed connection pads.
This type of device has certain limits insofar as a pixelization of the sensors is necessary to produce contacts on amorphous silicon. In fact, this pixelization constitutes a meticulous and costly step, making use of the photolithography technology, in addition to complex mating alignment equipment. Moreover, this mating is only effective via the connection pads, and this significantly affects the mechanical strength and reliability of the overall sensor. Besides, the preparation of electrodes in contact with the sensor material incurs a risk of local damage to this material.
Finally, document JP2002-334983 describes an imager comprising a transparent substrate associated with a transparent conductor and a photoconductor, provided at its terminals with an electron hole blocking layer, that is, a layer for blocking electrical carriers, and an electronic blocking layer. Also mentioned is a resistive protection layer prepared from an amorphous half-conductive material such as for example a-Se or a-Si. Conductive hybridization beads made from indium, provide the mating function, and connect the assembly thus formed to electrodes arranged on a read circuit. The overall assembly is laminated.
It appears from this structure that it is necessary to hybridize the read circuit with a soft material, in this case indium, in order to carry out the lamination.
In the absence of bonding layers on the resistive protection layer of amorphous half-conductor, it turns out that the overall mechanical strength is very low.
In short, we do not have today a technology for electrical interconnection between a sensing layer and a read circuit overcoming the problems of accuracy and performance, and serving to produce an assembly having reliable mechanical strength, long service life, and also simple to use, and easily reparable according to certain embodiments.
The present invention therefore relates to a device for detecting electromagnetic radiation and not having the drawbacks of the prior art sensors.